The invention relates generally to devices used for tensile testing of cables, and in particular to a tensile testing device for use with electrical cables and other non-standard composite cables in which elongation data of the entire sample is measured.
Mechanical properties of electrical power and signal cables found on precision spacecraft or other systems must be known if accurate structural models of cabled structures are desired. Until recently electrical cables were not considered an important contributor to the overall dynamics of cabled structures. However, the mass of cables relative to the overall structure mass has increased significantly over the last couple of decades as the complexity of structures increased. This trend motivated attempts to develop modeling tools able to accurately capture the dynamics of cabled structures. Cables are composite structures of individual wires twisted together with large fabrication variability and consequently development of analytical models based on properties of individual components is not a feasible approach. The experimental determination of the mechanical properties of cables through tensile (axial) and lateral tests is much more effective.
The standard tensile test device uses an extensometer mounted directly on the specimen as seen in FIG. 1. The ends of the specimen are pulled apart by the testing frame and load F is measured with a load cell. The extensometer is mounted directly on the specimen under test and measures the specimen's elongation between its two knife edges. The resulting stress measurement can then be reasonably extrapolated for the entire specimen's length for isotropic structures. Testing of composite electrical cables in this manner is not possible mainly because the extensometer cannot be placed directly on the sample. Placing the extensometer directly on the sample leads to erroneous data as the cable surface is not uniform and knife edges can slip. Additionally, cables are not isotropic structures and tend to twist during testing due to their construction. Therefore, strain measurements on a short cable length may not be accurate for the whole cable length. However, if strain is calculated based on the elongation of the entire sample, which must be long enough to be representative, a more reliable value is obtained. One way to measure elongation of the entire sample is by recording the displacement of the testing frame's cross-head displacement, but this leads to erroneous results because of the compliance associated with the fixtures (grips, connecting elements, etc). The present invention is designed to overcome these limitations.